Father Christmas is about to squeeze down chimneys with his sack of presents for children. But many cultures have a different version of the mystery visitor at night - and it's often a sinister one. Here are five examples.

Gryla, Iceland

Gryla is one of many female trolls who live high up in the mountains of Iceland.

Her story goes back to pagan times, but in more recent centuries she has become part of Christmas - making the trip down to the towns and cities, searching for naughty children.

She returns to her cave with a bag stuffed full of crying youngsters, whom she boils alive and gobbles up.

She has 13 sons, the Yule Lads, who also do their bit to harass Icelandic families in the 13 days up to Christmas - although recently they have become a little better behaved, and leave gifts in shoes too.

Gryla has hooves and horns on her head, 13 tails and a very big nose complete with warts.

She is in a perpetual bad mood, mainly because she is always hungry. And it's not only children who attract her ire - her first two husbands bored her so much she ate them.

Nightrunners, Kenya

Nightrunners are normal human beings during the day - they can be anyone in the community.

Why do these stories exist?

William Gray, Sussex Centre for Folklore, Fairy Tales and Fantasy

They often take place at twilight, between worlds. They evoke the idea that we all carry within us traces of an older, more primitive, way of being, in which we take what we want, when we want it - a part of ourselves we must give up.

Folklore and oral stories were told to the group. Children weren't separated out. So stories had to connect with everyone.

For us, what becomes important is a literal "scientific" truth - whether or not there are, in fact, any of these strange creatures. But in the past, people took it for granted that they were around.

They cared about what these creatures were telling them - what their dealings with these creatures meant, and what they symbolised.

But at night, they are transformed. Their hair is wild, their eyes surrounded by deep, black rings and they are stark naked.

They run through the village, banging on doors and throwing stones on roofs.

No-one can catch them because Nightrunners can fly, and disappear.

Sometimes they dig up the graves of the dead, or - if they don't feel like digging - they use magic to make coffins pop up out of the ground.

They take the bones for sorcery and eat the flesh.

Disturbing a Nightrunner is not a good idea.

They might use their powers to confuse your mind, perhaps even forcing you to come along for the exhumations…

The Sandman, Northern Europe

The Sandman is an age-old figure from northern European folklore who alternates between being a force for good and evil.

In some stories, this ghostly character visits children as they are tucked in at bedtime, sprinkling magic dust or sand from his special sack into their eyes to send them off into dreamland.

Sometimes he has a pair of umbrellas - one with lovely pictures on the underside for good children, and one with a black underside for bad children, to stop them dreaming.

But in other versions of the story, he is far more sinister.

In German author ETA Hoffman's version, written in the 19th Century, his sand makes children's eyes pop out and he fills his sack with the bloody eyeballs, ready to take them back to his owl-like children who live on the moon.

El Cuco , Spain, Portugal and Latin America

El Cuco is a shadowy monster with a penchant for eating disobedient children.

It hides under beds or inside wardrobes of children who refuse to sleep, eat or stay away from dangerous places.

The more naughty the behaviour of the child, the more hungry El Cuco becomes.

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Once caught, the victim is devoured or simply disappears, never to be seen or heard from again.

This shape-shifting figure takes many forms.

Sometimes El Cuco is depicted as a cloaked man, sometimes a fierce female dragon and sometimes a hairy monster or goblin.

Usually it has red eyes, and sometimes a red glowing ear. It is also known as El Coco and El Cucuy. Some stories have the monster as a child who suffered terrible violence and returns from the dead to terrorise the living.

Zwarte Piet, the Netherlands

Zwarte Piet or Black Pete is the side-kick of Sinterklaas, the Dutch St Nicholas, who comes on 5 December, travelling from his home in Spain, on a steamboat called Madrid.

Until the 19th Century, Sinterklaas did his own dirty work, bringing good children presents, but taking bad ones away in his sack for re-education and a beating.

But in 1850, children's author Jan Schenkman drew him with a black servant, who later became known as Zwarte Piet. It is now Zwarte Piet's job to go down the chimney to deliver presents and catch the less fortunate children.

Zwarte Piet has become increasingly controversial in the Netherlands due to a tradition of painting on a clownish black face, which many people find offensive.

Over the years several solutions have been tried - red-faced Piet, blue-faced Piet and Rainbow Piet - but none has worked because they terrified children or left adults puzzled.